Optical waveguide device and manufacturing method of the same

ABSTRACT

An optical waveguide device comprises a first transparent dielectric substrate having a predetermined index of refraction and a predetermined coefficient of thermal expansion, and a second transparent dielectric substrate having the same index of refraction and coefficient of thermal expansion as the first transparent dielectric substrate. An intervening layer having an index of refraction smaller than the index of refraction of the first and second transparent dielectric substrates is interposed between the first and second transparent dielectric substrates. An optical waveguide path is formed in at least either of the first and second transparent dielectric substrates.

This application is a division of application Ser. No. 08/087,436 filedJul. 8, 1993, now U.S. Pat. No. 5,373,579.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to a structure for improving theperformance of various optical devices used for light intensitymodulation, light switching, plane of polarization control, propagationmode control, optical phase matching control for second harmonicgeneration and other optical controls, and manufacturing method of thesame.

2. Description of the Prior Art

Optical waveguide devices, having been conventional developed, haveoptical waveguide paths formed on transparent dielectric substrates(such as lithium niobate (LiNbO₃)). These optical waveguide paths arevariously shaped and associated with appropriate electrodes to controlor modulate lights passing through therein by using electrooptic effect.For example, R. Alferness discloses this type of optical waveguidemodulator in the paper, "Waveguide Electrooptic Modulators", IEEETransactions on Microwave and Techniques, Vol. MTT-30, No.8,1121-1137(1982). Furthermore, I. Kaminow discloses various manufacturingmethods for this kind of optical waveguide device in the paper, "OpticalWaveguide Modulators" IEEE Transactions on Microwave and Techniques,Vol. MTT-23, No.1, 57-70(1975). These papers disclose various opticalwaveguide devices.

In one such manufacturing method, lithium niobate or lithium tantalateis heat-treated at a high temperature to modify the refractive index ofthe material by out-diffusing the lithium. Alternatively, a metallicfilm of, for example, titanium is formed by vacuum evaporation andthermally diffused at a high temperature to raise the refractive indexof the diffused area slightly above that of the surrounding area. Ineither case, the difference in refractive indices is used to confinelight. An example of a Mach-Zehnder type optical modulator using atitanium diffusion is described in Unexamined Japanese PatentApplication No. 63-261219/1988.

In another method, a metallic mask is formed over the specified areasand a proton-ion exchange is induced in phosphoric acid at 200° C. to300° C., partially modifying the refractive index and forming theoptical waveguide. Manufacturing methods that rely on out-diffusion,thermal diffusion, or ion exchange from the surface all form the opticalwaveguide by means of diffusion from the surface. The cross section ofthe optical waveguide is therefore necessarily determined by thediffusion process, resulting in numerous problems.

One of the biggest problems is coupling loss between the opticalwaveguide and the optical fiber. While the cross section of an opticalfiber is circular, the shape of the most conventional optical waveguideis roughly an inverted triangle due to the formation of the waveguide bydiffusion from the surface. Because the strength of the guided light isgreatest near the surface, optical coupling with the optical fiber ispoor, resulting in significant loss. Reducing this optical coupling lossis therefore an extremely important topic in optical waveguide devicedesign.

Another problem caused by diffusion processing is greater opticalpropagation loss after diffusion processing than before. With titaniumdiffusion optical waveguide, for example, propagating loss of more than1 dB/cm normally occurs. Reducing propagating loss is therefore anothermajor topic in optical waveguide device design.

A third problem is the increase in optical damage resulting fromdiffusion processing. Optical damage refers the increase in propagationloss over time when a high intensity light source or a short wavelengthlight source is input to a diffusion-type optical waveguide. This isbelieved to be caused by the diffusion of ions in the optical waveguideresulting in increased trapping of electrons in the optical waveguide.

It should be noted that methods for forming an optical waveguide withoutrelying on diffusion processing have been described. One of these isdescribed by Kaminow (see above reference). In this method, lithiumniobate crystals are grown on top of a lithium tantalate layer, or alithium niobate thin-film is formed by sputtering on top of a lithiumniobate or lithium tantalate layer, and the optical waveguide is formedin this lithium niobate top layer. A similar method is described inUnexamined Japanese Patent Application No. 52-23355/1977. This methodalso forms an epitaxial growth lithium niobate top layer over asubstrate of lithium tantalate (e.g.) using liquid phase, gas phase,fusion, or other method, and forms the optical waveguide in this toplayer. There are, however, several problems with these optical waveguideformation methods using such thin-film crystal growth technologies.First, it is difficult to obtain a thick-film in epitaxial growth film,and productivity is accordingly poor, because of the growth speed andflaws occurring in the crystals while being grown. In addition, thecoupling characteristics of a thin film less than 5 micron thick with anoptical fiber having a core diameter of approximately 10 microns arealso poor. (The fiber core being where the light is confined.)

Productivity is further hampered because a good quality single crystalthin-film cannot be obtained unless the lattice constants of thethin-film is essentially the same as those of the substrate. It istherefore extremely difficult to form a good lithium niobate thin-filmon a lithium tantalate substrate, and a mixed niobium-tantalum crystalfilm is often used. Pure lithium niobate, however, offers superioroverall optical waveguide characteristics when compared with a mixedcrystal film.

To increase the thickness of the growth layer, it may be possible to usethe same material between the growth layer and the substrate. But, thegrowth layer and substrate if made by the same material will have thesame crystal orientation. Due to the same crystal orientation, nosatisfactory difference will be obtained in the indices of refractionbetween the growth layer and substrate.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide a noveloptical waveguide device and a manufacturing method of the same which iscapable of reducing coupling loss, propagation loss, and optical damagewith respect to an ordinary optical fiber.

In order to attain above object, a first aspect of the present inventionprovides an optical waveguide device comprising: a first transparentdielectric substrate having a predetermined index of refraction and apredetermined coefficient of thermal expansion; a second transparentdielectric substrate having the same index of refraction and coefficientof thermal expansion as said first transparent dielectric substrate; anintervening layer having an index of refraction smaller than the indexof refraction of said first and second transparent dielectricsubstrates, said intervening layer being interposed between said firstand second transparent dielectric substrates; and an optical waveguidepath formed in at least either of said first and second transparentdielectric substrates.

Furthermore, a second aspect of the present invention provides anoptical waveguide device comprising: a first transparent dielectricsubstrate having a predetermined index of refraction; a secondtransparent dielectric substrate having an index of refraction largerthan said first transparent dielectric substrate, said secondtransparent dielectric substrate being directly connected with saidfirst transparent dielectric substrate; and an optical waveguide pathformed in said second transparent dielectric substrate.

Still further, a third aspect of the present invention provides anoptical waveguide device comprising: a first transparent dielectricsubstrate having a predetermined index of refraction; a secondtransparent dielectric substrate having an index of refraction largerthan said first transparent dielectric substrate; an intervening layerinterposed between said first and second transparent dielectricsubstrates; and an optical waveguide path formed in said secondtransparent dielectric substrate.

Moreover, a fourth aspect of the present invention provides an opticalwaveguide device comprising: a glass substrate having a predeterminedindex of refraction; a transparent dielectric substrate having an indexof refraction larger than said glass substrate, said transparentdielectric substrate being directly connected with said glass substrate;and an optical waveguide path formed in said transparent dielectricsubstrate.

Yet further a fifth aspect of the present invention provides an opticalwaveguide device comprising: a glass substrate having a predeterminedindex of refraction; a transparent dielectric substrate having an indexof refraction larger than said glass substrate; an intervening layerinterposed between said glass substrate and said transparent dielectricsubstrate; and an optical waveguide path formed in said transparentdielectric substrate.

The above and other objects, features and advantages of the presentinvention will become more apparent from the following detaileddescription which is to be read in conjunction with the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view showing an optical waveguide device adoptedto an optical modulator in accordance with a first embodiment of thepresent invention;

FIG. 2 is a cross-sectional view showing the optical waveguide device ofthe first embodiment, taken along a line A--A of FIG. 1;

FIGS. 3(a)-3(h) are views showing a first example of the method ofmanufacturing the optical waveguide device, by which the opticalwaveguide device of the first embodiment is manufactured;

FIGS. 4(a)-4(f) are views showing a second example of the method ofmanufacturing the optical waveguide device, by which the opticalwaveguide device of the first embodiment is manufactured;

FIGS. 5(a)-5(h) are views showing a third example of the method ofmanufacturing the optical waveguide device, by which the opticalwaveguide device of the first embodiment is manufactured;

FIGS. 6(a)-6(f) are views showing a fourth example of the method ofmanufacturing the optical waveguide device, by which the opticalwaveguide device of the first embodiment is manufactured;

FIG. 7 is a perspective view showing an optical waveguide device adoptedto an optical modulator in accordance with a second embodiment of thepresent invention;

FIG. 8 is a cross-sectional view showing the optical waveguide device ofthe second embodiment, taken along a line B--B of FIG. 7;

FIG. 9 is a perspective view showing an optical waveguide device adoptedto an optical modulator in accordance with a third embodiment of thepresent invention;

FIG. 10 is a perspective view showing an optical waveguide deviceadopted to an optical modulator in accordance with a fourth embodimentof the present invention;

FIGS. 11(a)-11(g) are views showing a fifth example of the method ofmanufacturing the optical waveguide device, by which the opticalwaveguide device of the second embodiment is manufactured;

FIGS. 12(a)-12(g) are views showing a sixth example of the method ofmanufacturing the optical waveguide device, by which the opticalwaveguide device of the third embodiment is manufactured;

FIGS. 13(a)-13(h) are views showing a seventh example of the method ofmanufacturing the optical waveguide device, by which the opticalwaveguide device of the fourth embodiment is manufactured;

FIG. 14 is a perspective view showing an optical waveguide deviceadopted to an optical modulator in accordance with a fifth embodiment ofthe present invention;

FIG. 15 is a cross-sectional view showing the optical waveguide deviceof the fifth embodiment, taken along a line C--C of FIG. 14;

FIG. 16 is a perspective view showing an optical waveguide deviceadopted to an optical modulator in accordance with a sixth embodiment ofthe present invention;

FIG. 17 is a perspective view showing an optical waveguide deviceadopted to an optical modulator in accordance with a seventh embodimentof the present invention;

FIGS. 18(a)-18(g) are views showing an eighth example of the method ofmanufacturing the optical waveguide device, by which the opticalwaveguide device of the fifth embodiment is manufactured;

FIGS. 19(a)-19(g) are views showing a ninth example of the method ofmanufacturing the optical waveguide device, by which the opticalwaveguide device of the sixth embodiment is manufactured; and

FIGS. 20(a)-20(h) are views showing a tenth example of the method ofmanufacturing the optical waveguide device, by which the opticalwaveguide device of the seventh embodiment is manufactured.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Hereinafter, preferred embodiments of the present invention will bedescribed in detail with reference to accompanying drawings.

1. FIRST EMBODIMENT

FIGS. 1 and 2 show the first embodiment of the present invention, whichis applied to an optical modulator. In the drawings, a reference numeral1 represents a first transparent dielectric substrate havingpredetermined index of refraction and coefficient of thermal expansion.A reference numeral 2 represents a second transparent dielectricsubstrate having the same index of refraction and coefficient of thermalexpansion as the first transparent dielectric substrate 1. This secondtransparent dielectric substrate 2 is formed thin compared with thefirst transparent dielectric substrate 1. A reference numeral 3represents an inlet or outlet portion of an optical waveguide path,formed on the second transparent dielectric substrate 2 at opposite endsthereof. An intermediate portion of the optical waveguide path betweenthe inlet and outlet portions 3, 3 is split symmetrically into two atthe center of the second transparent dielectric substrate 2. One is afirst bifurcated optical waveguide path 4, and the other is a secondbifurcated optical waveguide path 5. These two bifurcated opticalwaveguide paths 4 and 5 are formed in parallel with each other. Atopposite sides of the second bifurcated optical waveguide path 5, thereis provided a pair of electrodes 8 and 7 of aluminum. An interveninglayer 9, being a dielectric material and having an index of refractionsmaller than that of the first and second transparent dielectricsubstrates 1, 2, interposes between the first and second transparentdielectric substrates 1, 2.

The first and second bifurcated optical waveguide paths 4, 5, having thesame cross section of a trapezoid, constitute so-called ridges. Thecross section of the first and second bifurcated waveguide paths 4, 5 isthe same as that of the inlet and outlet portions 3, 3 of the opticalwaveguide path. A reference numeral 8 represents a light propagatingregion of an optical waveguide path.

The construction disclosed here is referred to as Mach-Zehnder type, inwhich a light entered from the inlet portion 3 is introduced intobifurcated two intermediate paths 4, 5, one 5 of which is applied with acertain voltage to change its index of refraction by usingelectrooptical effect. A propagation speed of the light, passing throughthe bifurcated waveguide path 5, changes due to the change of the indexof refraction of the bifurcated waveguide path 5. With this change ofpropagation speed, two lights propagating in the first and secondbifurcated intermediate paths 4, 5 are modified to be out of phase witheach other. These two lights are merged into one again in the outletportion 3. Thus merged light has a modified intensity compared with theoriginal one. Thus, the optical modulation can be carried out in theMach-Zehnder type optical waveguide device.

A monocrystal lithium niobate and a monocrystal lithium tantalaterespectively having a large electrooptical effect are selectively usedin this embodiment as the material constituting the first and secondtransparent dielectric substrates 1 and 2. The monocrystal lithiumniobate has an index of refraction of 2.29 with respect to an ordinarylight. On the other hand, the monocrystal lithium tantalate has an indexof refraction of 2.18 with respect to the ordinary light. It should benoted that these indices of refraction tend to slightly vary based onrespective crystal orientations. This embodiment selected a pair oftransparent dielectric substrates made of the same material having thesame crystal orientation. With this combination, the coefficients of thefirst and second transparent dielectric substrates become identical witheach other.

The intervening layer 9 preferably comprises a component selected fromthe group consisting of silicon, silicon compound, and metallic oxide.Silicon dioxide and silicon nitride are selectively used in thisembodiment as the material constituting the intervening layer 9. Indicesof the silicon dioxide and silicon nitride are approximately 1.5 and2.0, respectively, with respect to the ordinary light. Namely, the indexof refraction of the intervening layer 9 can be made smaller than thatof the first and second transparent dielectric substrates 1, 2, ifsilicon dioxide or silicon nitride is adopted as the intervening layer 9while lithium niobate or lithium tantalate is selected as the first andsecond transparent dielectric substrates 1, 2.

In order to confine the propagation of lights or electromagnetic wavesin either of the first and second transparent dielectric substrates 1,2, it is necessary to increase a thickness of the intervening layer 9 toa certain degree. In practice, the thickness of the intervening layer 9is selected 2 μm in the case where the first and second transparentdielectric substrates 1, 2 are 400 μm and 7 μm, respectively, inthickness. The thickness of the intervening layer 9 should be selectedto be equal to or more than a wavelength of a light to be used.

Furthermore, the ridge configuration of this embodiment is effective inthe localized confinement of the propagation of lights orelectromagnetic waves. Because, the portion right beneath the ridge hasa larger effective index of refraction compared with other portion. Inthis embodiment, the specific dimension of the ridge is determined asfollows: A height of the ridge is 2 μm; a width of the waveguide path is7 μm; a length of the bifurcated portion of the waveguide path is 2 cm;and an whole length of the waveguide path is 3 cm.

The ridge configuration is further advantageous in that a center of thepropagation, i.e. a portion at which an intensity of light orelectromagnetic wave is strongest, is positioned almost identically withthe center of the waveguide path formed beneath the ridge. Moreover, theshape of the propagation of lights or electromagnetic waves is similarto a circle. As the inlet and outlet portion 3, 3 of the waveguide pathhave the same circular constructions, the coupling efficiency withrespect to an optical fiber, which has generally a circular core(approximately 10 μm in a diameter), is greatly improved.

In fact, a coupling loss with respect to the optical fiber was less than0.3-0.5 dB at one side in the case where an adhesive material whoseindex of refraction is appropriately adjusted is used to connect theoptical fiber and the waveguide device of this embodiment. On thecontrary, this coupling loss increases to a value equal to or more than1.0 dB if the conventional diffused type optical waveguide device madeof lithium niobate or lithium tantalate is used.

Still further, the propagation loss of lights or electromagnetic wavescan be suppressed to the least value because the transparent dielectricsubstrates used in this embodiment are made of pure monocrystal, whichrequires no ion-diffusion treatment. In more detail, a value equal to orless than 0.5 dB/cm was easily attained in the propagation loss.

Furthermore no optical damage was found, although the optical damage wasmeasured by increasing the intensity of incident light from 0 dBm to 20dBm. This good result is considered to be derived from the excellentproperty of the monocrystal substrate having very small electronic trap.

Yet further, the performance as an optical modulator was as well as theconventional device.

A light used in the measurement was a wavelength of 1.3 μm.

No adhesive material of organic substance was used for the connectionbetween the intervening layer and the substrate. Instead, intermolecularforce and electrostatic force are utilized for tightly bonding theseinorganic substances. This connection is advantageous in productivitybecause it is easy and quick to complete the connection. But, such adirect-type connection is weak against temperature change. If thecoefficients of two substrates are different from each other, distortionwill occur in the connecting area with increasing temperature. Thismeans that no sufficient connecting strength would be obtained.Therefore, it is important to improve the durability or reliabilityagainst temperature change. For this purpose, it is preferable to usesubstrates having the same coefficients of thermal expansion. Secondly,it is further preferable that the crystal orientations of thesesubstrates are identical with each other. Because, the coefficient ofthermal expansion may slightly vary depending on the crystalorientation. As the substrates used in this embodiment are made of thesame material having the same crystal orientation, its direct-typeconnection is firm and therefore reliable.

Barium titanate, potassium niobate, and potassium titanophosphate areprospective materials which have similar electrooptical effect and canbe equivalently used as a transparent dielectric substrate of thepresent invention instead of lithium niobate or lithium tantalate. Theindices of refraction of barium titanate, potassium niobate, andpotassium titanophosphate are 2.4, 2.2, and 1.7, respectively.

Various metallic oxide including zinc oxide, aluminum oxide, indiumoxide, various soda glass, and optical glass can be used as theintervening layer of the present invention instead of silicon dioxide orsilicon nitride. The indices of refraction of zinc oxide, aluminumoxide, indium oxide are 2.0, 1.6, 2.0, respectively. That of varioussoda glass is approximately 1.5, and the optical glass is 1.5-1.8.

1.1. FIRST METHOD

A first example of the method of manufacturing the optical waveguidedevice of the first embodiment will be explained below with reference toFIGS. 3(a)-3(h).

First of all, first and second transparent dielectric substrates 1 and2, which are identical with each other in index of refraction andcoefficient of thermal expansion, are prepared. The surfaces of thesefirst and second transparent dielectric substrates 1 and 2, which havebeen ground and finished as mirror surfaces beforehand, are smoothed andcleaned. The intervening layer 9 of silicon dioxide or silicon nitride,which has an index of refraction smaller than that of the first andsecond transparent dielectric substrates 1 and 2, is grown on at leasteither of the smoothed and cleaned surfaces of the first and secondtransparent dielectric substrates 1 and 2, as shown in FIG. 3(a). Aconventional growth method such as chemical vapor deposition (CVD) orspattering is used to form this intervening layer 9.

Thereafter, both surfaces to be bonded through the intervening layer 9,i.e. the upper surface of the intervening layer 9 and the upper surfaceof the first transparent dielectric substrate 1 in the drawing, areadopted a hydrophilic treatment. In detail, they are soaked in asolution consisting of hydrogen peroxide (H₂ O₂)--ammonia(NH₃)--water(H₂ O), as shown in FIG. 3(b). Subsequently, the surfaces tobe bonded are watered as shown in FIG. 3(c). This watering treatmentallows components constituting water to adhere on the surfaces of thesubstrate 1 and intervening layer 9. Immediately after the wateringtreatment, both surfaces of the substrate 1 and the intervening layer 9are uniformly connected together as shown in FIG. 3(d) without using anyorganic adhesive material. Namely, this connection is direct-typeconnection achieved by the bonding power of water, hydroxyl group,hydrogen ions adhering on the surface of the substrate 1 or theintervening layer 9.

In turn, a heat treatment is adopted to the assembly of the substrates1, 2 and the intervening layer 9 interposing therebetween, as shown inFIG. 3(e). The temperature is increased to a point equal to or more than100° C. This heat treatment ensures the connecting strength to befurther increased. The reason is considered as follows: As the heattreatment progresses, water and hydrogen are gradually removed off theconnecting surfaces. Then, remaining oxygen reacts with the component ofthe transparent dielectric substrates 1, 2 and, as a result, a firmconnection is finally obtained.

Next, in order to form an optical waveguide path, either of thesubstrates, i.e. the second transparent dielectric substrate 2 in thedrawing, is thinned by the mechanical grind and etching as shown in FIG.3(f). A thickness of the second substrate 2 is reduced to a level of7-10 μm. On the surface of the second substrate 2, there is then formedan etching mask corresponding to the Mach-Zehnder type optical waveguidestructure shown in FIG. 1 by using photolithography. Thus, the portionother than ridges 4, 5 of the optical waveguide structure is removed offby etching as shown in FIG. 3(g). The depth formed by this etching is 2μm. Chrome-gold is used as the mask. The etchant used here containshydrofluoric acid. Thereafter, the mask is removed and a pair ofelectrodes 6, 7 is formed by the ordinary photolithography and etchingtechniques as shown in FIG. 3(h). In this manner, according to thisfirst example of the manufacturing method, the optical waveguide deviceof the first embodiment can be obtained.

1.2. SECOND METHOD

A second example of the method of manufacturing the optical waveguidedevice of the first embodiment will be explained below with reference toFIGS. 4(a)-4(f).

First of all, first and second transparent dielectric substrates 1 and2, which are identical with each other in index of refraction andcoefficient of thermal expansion, are prepared. The surfaces of thesefirst and second transparent dielectric substrates 1 and 2, which havebeen ground and finished as mirror surfaces beforehand, are smoothed andcleaned. The intervening layer 9 of silicon dioxide or silicon nitride,which has an index of refraction smaller than that of the first andsecond transparent dielectric substrates 1 and 2, is grown on at leasteither of the smoothed and cleaned surfaces of the first and secondtransparent dielectric substrates 1 and 2, as shown in FIG. 4(a). Aconventional growth method such as chemical vapor deposition (CVD) orspattering is used to form this intervening layer 9.

Thereafter, both surfaces to be bonded through the intervening layer 9,i.e. the upper surface of the intervening layer 9 and the upper surfaceof the first transparent dielectric substrate 1 in the drawing, areuniformly connected together as shown in FIG. 4(b) without using anyorganic adhesive material.

A direct voltage of 100-2,000 V is applied to the assembly of thesubstrates 1, 2 and the intervening layer 9 interposing therebetween, asshown in FIG. 4(c). The DC voltage generally induces a flow of ions onthe connecting surfaces, which is considered to generate electrostaticbonding force. This connection will be accomplished more quickly if aheat treatment is additionally adopted. In any case, the usage of DCvoltage ensures a direct-type connection at a lower temperature comparedwith the above-described first method.

Next, in order to form an optical waveguide path, either of thesubstrates, i.e. the second transparent dielectric substrate 2 in thedrawing, is thinned by the mechanical grind and etching as shown in FIG.4(d). A thickness of the second substrate 2 is reduced to a level of7-10 μm. On the surface of the second substrate 2, there is then formedan etching mask corresponding to the Mach-Zehnder type optical waveguidestructure shown in FIG. 1 by using photolithography. Thus, the portionother than ridges 4, 5 of the optical waveguide structure is removed offby etching as shown in FIG. 4(e). The depth formed by this etching is 2μm. Chrome-gold is used as the mask. The etchant used here containshydrofluoric acid. Thereafter, the mask is removed and a pair ofelectrodes 6, 7 is formed by the ordinary photolithography and etchingtechniques as shown in FIG. 4(f). In this manner, according to thissecond example of the manufacturing method, the optical waveguide deviceof the first embodiment can be obtained.

1.3. THIRD METHOD

A third example of the method of manufacturing the optical waveguidedevice of the first embodiment will be explained below with reference toFIGS. 5(a)-5(h).

First of all, first and second transparent dielectric substrates 1 and2, which are identical with each other in index of refraction andcoefficient of thermal expansion, are prepared. The surfaces of thesefirst and second transparent dielectric substrates 1 and 2, which havebeen ground and finished as mirror surfaces beforehand, are smoothed andcleaned. The intervening layer 9 of silicon dioxide or silicon nitride,which has an index of refraction smaller than that of the first andsecond transparent dielectric substrates 1 and 2, is grown on at leasteither of the smoothed and cleaned surfaces of the first and secondtransparent dielectric substrates 1 and 2, as shown in FIG. 5(a). Aconventional growth method such as chemical vapor deposition (CVD) orspattering is used to form this intervening layer 9.

Thereafter, both surfaces to be bonded through the intervening layer 9,i.e. the upper surface of the intervening layer 9 and the upper surfaceof the first transparent dielectric substrate 1 in the drawing, areadopted a hydrophilic treatment. In detail, they are soaked in asolution consisting of hydrogen peroxide (H₂ O₂)--ammonia(NH₃)--water(H₂ O), as shown in FIG. 5(b). Subsequently, the surfaces tobe bonded are watered as shown in FIG. 5(c). This watering treatmentallows components constituting water to adhere on the surfaces of thesubstrate 1 and intervening layer 9. Immediately after the wateringtreatment, both surfaces of the substrate 1 and the intervening layer 9are uniformly connected together as shown in FIG. 5(d) without using anyorganic adhesive material. Namely, this connection is direct-typeconnection achieved by water, hydroxyl group, hydrogen adhering on thesurface of the substrate 1 or the intervening layer 9.

A direct voltage of 100-2,000 V is applied to the assembly of thesubstrates 1, 2 and the intervening layer 9 interposing therebetween, asshown in FIG. 5(e). This connection will be accomplished more quickly ifa heat treatment is additionally adopted. In any case, the usage of DCvoltage ensures a direct-type connection at a lower temperature comparedwith the above-described first method.

Next, in order to form an optical waveguide path, either of thesubstrates, i.e. the second transparent dielectric substrate 2 in thedrawing, is thinned by the mechanical grind and etching as shown in FIG.5(f). A thickness of the second substrate 2 is reduced to a level of7-10 μm. On the surface of the second substrate 2, there is then formedan etching mask corresponding to the Mach-Zehnder type optical waveguidestructure shown in FIG. 1 by using photolithography. Thus, the portionother than ridges 4, 5 of the optical waveguide structure is removed offby etching as shown in FIG. 5(g). The depth formed by this etching is 2μm. Chrome-gold is used as the mask. The etchant used here containshydrofluoric acid. Thereafter, the mask is removed and a pair ofelectrodes 6, 7 is formed by the ordinary photolithography and etchingtechniques as shown in FIG. 5(h). In this manner, according to thisthird example of the manufacturing method, the optical waveguide deviceof the first embodiment can be obtained.

1.4. FOURTH METHOD

A fourth example of the method of manufacturing the optical waveguidedevice of the first embodiment will be explained below with reference toFIGS. 6(a)-6(f).

First of all, first and second transparent dielectric substrates 1 and2, which are identical with each other in index of refraction andcoefficient of thermal expansion, are prepared. The surfaces of thesefirst and second transparent dielectric substrates 1 and 2, which havebeen ground and finished as mirror surfaces beforehand, are smoothed andcleaned. A glass layer 9' having a low melting point, whose index ofrefraction is smaller than that of the first and second transparentdielectric substrates 1 and 2, is grown on at least either of thesmoothed and cleaned surfaces of the first and second transparentdielectric substrates 1 and 2, as shown in FIG. 6(a). A conventionalgrowth method such as chemical vapor deposition (CVD) or spattering isused to form this glass layer 9'. Thereafter, both surfaces to be bondedthrough the glass layer 9', i.e. the upper surface of the glass layer 9'and the upper surface of the first transparent dielectric substrate 1 inthe drawing, are uniformly connected together as shown in FIG. 6(b)without using any organic adhesive material.

In turn, a heat treatment is adopted to the assembly of the substrates1, 2 and the glass layer 9' interposing therebetween, as shown in FIG.6(c). The temperature is increased to or a point nearly equal to themelting point of this glass layer. It is preferable to select the glasslayer 9' from glass materials whose melting point are in a range of300°-800° C. This heat treatment ensures the strength of this directconnection to be increased.

If the temperature is increased to exceed the melting point of the glasslayer 9', perfect melting of glass layer 9' and subsequent firmconnection will be surely taken place. However, even if the temperaturedoes not exceed the melting point, satisfactory connection would beobtained by the softened glass. This will bring a merit of maintainingan original thickness of the glass layer.

Next, in order to form an optical waveguide path, either of thesubstrates, i.e. the second transparent dielectric substrate 2 in thedrawing, is thinned by the mechanical grind and etching as shown in FIG.6(d). A thickness of the second substrate 2 is reduced to a level of7-10 μm. On the surface of the second substrate 2, there is then formedan etching mask corresponding to the Mach-Zehnder type optical waveguidestructure shown in FIG. 1 by using photolithography. Thus, the portionother than ridges 4, 5 of the optical waveguide structure is removed offby etching as shown in FIG. 6(e). The depth formed by this etching is 2μm. Chrome-gold is used as the mask. The etchant used here containshydrofluoric acid. Thereafter, the mask is removed and a pair ofelectrodes 6, 7 is formed by the ordinary photolithography and etchingtechniques as shown in FIG. 6(f). In this manner, according to thisfourth example of the manufacturing method, the optical waveguide deviceof the first embodiment can be obtained.

2. SECOND EMBODIMENT

FIGS. 7 and 8 show the second embodiment of the present invention, whichis applied to an optical modulator. In FIG. 7, a reference numeral 11represents a first transparent dielectric substrate having apredetermined index of refraction. A reference numeral 12 represents asecond transparent dielectric substrate having an index of refractiondifferent from that of the first transparent dielectric substrate 11.The difference of the index of refraction between two substrates 11, 12can be obtained by changing an amount of impurity contained therein.This second transparent dielectric substrate 12 is formed thin comparedwith the first transparent dielectric substrate 11. A reference numeral13 represents an inlet or outlet portion of an optical waveguide path,formed on the second transparent dielectric substrate 12 at oppositeends thereof. An intermediate portion of the optical waveguide pathbetween the inlet and outlet portions 13, 13 is split symmetrically intotwo at the center of the second transparent dielectric substrate 12. Oneis a first bifurcated optical waveguide path 14, and the other is asecond bifurcated optical waveguide path 15. These two bifurcatedoptical waveguide paths 14 and 15 are formed in parallel with eachother. At opposite sides of the second bifurcated optical waveguide path15, there is provided a pair of electrodes 16 and 17 of aluminum.

The first and second bifurcated optical waveguide paths 14, 15, havingthe same cross section of a trapezoid, constitute so-called ridges. Thecross section of the first and second bifurcated waveguide paths 14, 15is the same as that of the inlet and outlet portions 13, 13 of theoptical waveguide path. A reference numeral 18 represents a lightpropagating region of an optical waveguide path.

The construction disclosed here is referred to as Mach-Zehnder type, inwhich a light entered from the inlet portion 13 is introduced intobifurcated two intermediate paths 14, 15, one 15 of which is appliedwith a certain voltage to change its index of refraction by usingelectrooptical effect. A propagation speed of the light, passing throughthe bifurcated waveguide path 15, changes due to the change of the indexof refraction of the bifurcated waveguide path 15. With this change ofpropagation speed, two lights propagating in the first and secondbifurcated intermediate paths 14, 15 are modified to be out of phasewith each other. These two lights are merged into one again in theoutlet portion 13. Thus merged light has a modified intensity comparedwith the original one. Thus, the optical modulation can be carried outin the Mach-Zehnder type optical waveguide device.

A monocrystal lithium niobate and a monocrystal lithium tantalaterespectively having a large electrooptical effect are selectively usedin this embodiment as the material constituting the first and secondtransparent dielectric substrates 1 and 2. The monocrystal lithiumniobate has an index of refraction of 2.29 with respect to an ordinarylight. On the other hand, the monocrystal lithium tantalate has an indexof refraction of 2.18 with respect to the ordinary light.

If there is a difference equal to or more than 0.01 in index ofrefraction, it is possible to confine lights or electromagnetic waves ineither of two substrates which has a greater index of refraction. Inthis embodiment, the second transparent dielectric substrate 12 isadjusted to be larger than the first transparent dielectric substrate 11in index of refraction. In more detail, the first transparent dielectricsubstrate 11 contains impurity such as magnesium (Mg) of approximately10²⁰ /cm³. On the other hand, the second transparent dielectricsubstrate 12 contains no impurity. With this injection of impurity tothe first transparent dielectric substrate 11, a refractive indexdifference of 0.01 can be obtained between two substrates 11, 12. Thus,a light entered into the second transparent dielectric substrate 12 canbe confined in this thin layer. The thicknesses of the first and secondtransparent dielectric substrates 11, 12 are selected to 600 μm and 7μm, respectively.

Furthermore, the ridge configuration of this embodiment is effective inthe localized confinement of the propagation of lights orelectromagnetic waves. Because, the portion right beneath the ridge hasa larger effective index of refraction compared with other portion. Inthis embodiment, the specific dimension of the ridge is determined asfollows: A height of the ridge is 3 μm; a width of the waveguide path is7 μm; a length of the bifurcated portion of the waveguide path is 2 cm;and an whole length of the waveguide path is 3 cm.

The ridge configuration is further advantageous in that a center of thepropagation, i.e. a portion at which an intensity of light orelectromagnetic wave is strongest, is positioned almost identically withthe center of the waveguide path formed beneath the ridge. Moreover, theshape of the propagation of lights or electromagnetic waves is similarto a circle. As the inlet and outlet portion 13, 13 of the waveguidepath have the same circular constructions, the coupling efficiency withrespect to an optical fiber, which has generally a circular core(approximately 10 μm in a diameter), is greatly improved.

In fact, a coupling loss with respect to the optical fiber was less than0.3-0.5 dB at one side in the case where an adhesive material whoseindex of refraction is appropriately adjusted is used to connect theoptical fiber and the waveguide device of this embodiment.

Still further, the propagation loss of lights or electromagnetic wavescan be suppressed to the least value because the transparent dielectricsubstrates used in this embodiment are made of pure monocrystal, whichrequires no ion-diffusion treatment. In more detail, a value equal to orless than 0.5 dB/cm was easily attained in the propagation loss.

Furthermore no optical damage was found, although the optical damage wasmeasured by increasing the intensity of incident light from 0 dBm to 20dBm. This good result is considered to be derived from the excellentproperty of the monocrystal substrate having very small electronic trap.

A light used in the measurement was a wavelength of 1.3 μm.

Barium titanate, potassium niobate, and potassium titanophosphate areprospective materials which have similar electrooptical effect and canbe equivalently used as a transparent dielectric substrate of thepresent invention instead of lithium niobate or lithium tantalate. Theindices of refraction of barium titanate, potassium niobate, andpotassium titanophosphate are 2.4, 2.2, and 1.7, respectively.

2.1. FIFTH METHOD

An example of the method of manufacturing the optical waveguide deviceof the second embodiment will be explained below with reference to FIGS.11(a)-11(g).

First of all, first and second transparent dielectric substrates 11 and12, which are slightly different from each other in index of refraction,are prepared. The surfaces of these first and second transparentdielectric substrates 11 and 12, which have been ground and finished asmirror surfaces beforehand, are smoothed and cleaned. In practice,etchant containing hydrofluoric acid is used to clean the surfaces ofthe first and second transparent dielectric substrates 11 and 12 asshown in FIG. 11(a). Subsequently, the etched surfaces are watered asshown in FIG. 11(b). This watering treatment allows water componentssuch as hydroxyl group, hydrogen, oxygen to adhere on the surfaces ofthe substrates 11, 12. Immediately after the watering treatment, bothsurfaces of the substrates 11, 12 are uniformly connected together asshown in FIG. 11(c) without using any organic adhesive material. Namely,this connection is direct-type connection achieved by the bonding powerof hydroxyl group, hydrogen, oxygen ions adhering on the surface of thesubstrates 11, 12.

In turn, a heat treatment is adopted to the assembly of the substrates11, 12, as shown in FIG. 11(d). The temperature is increased to a rangeof 100°-1100° C. This heat treatment ensures the connecting strength tobe further increased. The reason is already described in the firstmethod.

Next, in order to form an optical waveguide path, either of thesubstrates, i.e. the second transparent dielectric substrate 12 in thedrawing, is thinned by the mechanical grind and etching as shown in FIG.11(e). A thickness of the second substrate 12 is reduced to a level of 7μm. On the surface of the second substrate 12, there is then formed anetching mask corresponding to the Mach-Zehnder type optical waveguidestructure shown in FIG. 7 by using photolithography. Thus, the portionother than ridges 14, 15 of the optical waveguide structure is removedoff by etching as shown in FIG. 11(f). The depth formed by this etchingis 3 μm. Chrome is used as the mask. The etchant used here containshydrofluoric acid. Thereafter, the mask is removed and a pair ofelectrodes 16, 17 is formed by the ordinary photolithography and etchingtechniques as shown in FIG. 11(g). In this manner, according to thisfifth example of the manufacturing method, the optical waveguide deviceof the second embodiment can be obtained.

3. THIRD EMBODIMENT

FIG. 9 shows the third embodiment of the present invention, which isapplied to an optical modulator. In FIG. 9, components suffixed by thereference numerals 11-17 are the same as those disclosed in FIG. 7 andtherefore will no more be explained. A reference numeral 19 represents aglass layer, which interposes between the first and second transparentdielectric substrates 11, 12.

The index of refraction of the glass layer 19 is approximately 1.5. Thethickness of the glass layer 19 is selected to be 100 nm to 1 μm, whichis far thin compared with that of the second transparent dielectricsubstrate 12. With this arrangement, it becomes possible to effectivelyconfine lights or electromagnetic waves in the second transparentdielectric substrate 12 because of the refractive index differencebetween the first and second substrates 11, 12 as described in thesecond embodiment.

Substantially the same function and effect as the second embodiment canbe obtained in this third embodiment. Especially almost identicalproperties will be obtained, if the thickness of the glass layer 19 isselected to be 0.5 μm.

3.1. SIXTH METHOD

An example of the method of manufacturing the optical waveguide deviceof the third embodiment will be explained below with reference to FIGS.12(a)-12(g).

First of all, first and second transparent dielectric substrates 11 and12, which are slightly different from each other in index of refraction,are prepared. The surfaces of these first and second transparentdielectric substrates 11 and 12, which have been ground and finished asmirror surfaces beforehand, are smoothed and cleaned. In practice,etchant containing hydrofluoric acid is used to clean the surfaces ofthe first and second transparent dielectric substrates 11 and 12 asshown in FIG. 12(a). Subsequently, a glass layer 19 is grown on at leasteither of the smoothed and cleaned surfaces of the first and secondtransparent dielectric substrates 11 and 12, as shown in FIG. 12(b). Aconventional growth method such as spattering is used to form this glasslayer 19. The thickness of the glass layer 19 is selected to be 0.3-0.6μm.

Then, both substrates 11, 12 are uniformly connected together throughthe glass layer 19 as shown in FIG. 12(c) without using any organicadhesive material.

In turn, a heat treatment is adopted to the assembly of the substrates11, 12 and the glass layer 19 interposing therebetween, as shown in FIG.12(d). The temperature is increased to a point nearly equal to themelting point of the glass layer 19. This heat treatment ensures theconnecting strength to be increased.

Next, in order to form an optical waveguide path, either of thesubstrates, i.e. the second transparent dielectric substrate 12 in thedrawing, is thinned by the mechanical grind and etching as shown in FIG.12(e). A thickness of the second substrate 12 is reduced to a level of 7μm. On the surface of the second substrate 12, there is then formed anetching mask corresponding to the Mach-Zehnder type optical waveguidestructure shown in FIG. 9 by using photolithography. Thus, the portionother than ridges 14, 15 of the optical waveguide structure is removedoff by etching as shown in FIG. 12(f). The depth formed by this etchingis 3 μm. Chrome is used as the mask. The etchant used here containshydrofluoric acid. Thereafter, the mask is removed and a pair ofelectrodes 16, 17 is formed by the ordinary photolithography and etchingtechniques as shown in FIG. 12(g). In this manner, according to thissixth example of the manufacturing method, the optical waveguide deviceof the third embodiment can be obtained.

4. FOURTH EMBODIMENT

FIG. 10 shows the fourth embodiment of the present invention, which isapplied to an optical modulator. In FIG. 10, components suffixed by thereference numerals 11-17 are the same as those disclosed in FIG. 7 andtherefore will no more be explained. A reference numeral 20 representsan intervening layer, which interposes between the first and secondtransparent dielectric substrates 11, 12.

The intervening layer 20 is constituted by a material selected from agroup consisting of silicon, silicon compound, and metallic oxide suchas polycrystal silicon, amorphous silicon, silicon oxide and siliconnitride. The index of refraction of the intervening layer 20 isdifferent from that of the transparent dielectric substrates 11, 12. Thethickness of the intervening layer 20 is selected to be 100 nm to 1 μm,which is far thin compared with that of the second transparentdielectric substrate 12. With this arrangement, it becomes possible toeffectively confine lights or electromagnetic waves in the secondtransparent dielectric substrate 12 because of the refractive indexdifference between the first and second substrates 11, 12 as describedin the second embodiment.

Substantially the same function and effect as the second embodiment canbe obtained in this fourth embodiment. Especially almost identicalproperties will be obtained, if the thickness of the intervening layer20 is selected to be 0.5 μm.

4.1. SEVENTH METHOD

An example of the method of manufacturing the optical waveguide deviceof the fourth embodiment will be explained below with reference to FIGS.13(a)-13(h).

First of all, first and second transparent dielectric substrates 11 and12, which are slightly different from each other in index of refraction,are prepared. The surfaces of these first and second transparentdielectric substrates 11 and 12, which have been ground and finished asmirror surfaces beforehand, are smoothed and cleaned, for example, byetching. Subsequently, the intervening layer 20 is grown on at leasteither of the smoothed and cleaned surfaces of the first and secondtransparent dielectric substrates 11 and 12, as shown in FIG. 13(a). Aconventional growth method such as chemical vapor deposition (CVD) orspattering is used to form this intervening layer 20. The thickness ofthe intervening layer 20 is selected to be 0.25-0.5 μm.

Thereafter, both surfaces to be bonded through the intervening layer 20,i.e. the upper surface of the intervening layer 20 and the upper surfaceof the first transparent dielectric substrate 11 in the drawing, areadopted a hydrophilic treatment, as shown in FIG. 13(b). Subsequently,the surfaces to be bonded are watered as shown in FIG. 13(c).Immediately after the watering treatment, both surfaces of the substrate11 and the intervening layer 20 are uniformly connected together asshown in FIG. 13(d) without using any organic adhesive material.

In turn, a heat treatment is adopted to the assembly of the substrates11, 12 and the intervening layer 20 interposing therebetween, as shownin FIG. 13(e). The temperature is increased to a range of 100°-1100° C.This heat treatment ensures the connecting strength to be increased.

Next, in order to form an optical waveguide path, either of thesubstrates, i.e. the second transparent dielectric substrate 12 in thedrawing, is thinned by the mechanical grind and etching as shown in FIG.13(f). A thickness of the second substrate 12 is reduced to a level of 7μm. On the surface of the second substrate 12, there is then formed anetching mask corresponding to the Mach-Zehnder type optical waveguidestructure shown in FIG. 10 by using photolithography. Thus, the portionother than ridges 14, 15 of the optical waveguide structure is removedoff by etching as shown in FIG. 13(g). The depth formed by this etchingis 3 μm. Chrome is used as the mask. The etchant used here containshydrofluoric acid. Thereafter, the mask is removed and a pair ofelectrodes 16, 17 is formed by the ordinary photolithography and etchingtechniques as shown in FIG. 13(h). In this manner, according to thisseventh example of the manufacturing method, the optical waveguidedevice of the fourth embodiment can be obtained.

5. FIFTH EMBODIMENT

FIGS. 14 and 15 show the fifth embodiment of the present invention,which is applied to an optical modulator. In FIG. 14, a referencenumeral 21 represents a glass substrate. A reference numeral 22represents a transparent dielectric substrate having an electroopticaleffect. This second transparent dielectric substrate 12 is formed thincompared with the glass substrate 21. A reference numeral 23 representsan inlet or outlet portion of an optical waveguide path, formed on thetransparent dielectric substrate 22 at opposite ends thereof. Anintermediate portion of the optical waveguide path between the inlet andoutlet portions 23, 23 is split symmetrically into two at the center ofthe transparent dielectric substrate 22. One is a first bifurcatedoptical waveguide path 24, and the other is a second bifurcated opticalwaveguide path 25. These two bifurcated optical waveguide paths 24 and25 are formed in parallel with each other. At opposite sides of thesecond bifurcated optical waveguide path 25, there is provided a pair ofelectrodes 28 and 27 of aluminum.

The first and second bifurcated optical waveguide paths 24, 25, havingthe same cross section of a trapezoid, constitute so-called ridges. Thecross section of the first and second bifurcated waveguide paths 24, 25is the same as that of the inlet and outlet portions 23, 23 of theoptical waveguide path. A reference numeral 28 represents a lightpropagating region of an optical waveguide path.

The construction disclosed here is referred to as Mach-Zehnder type, inwhich a light entered from the inlet portion 23 is introduced intobifurcated two intermediate paths 24, 25, one 25 of which is appliedwith a certain voltage to change its index of refraction by usingelectrooptical effect. A propagation speed of the light, passing throughthe bifurcated waveguide path 25, changes due to the change of the indexof refraction of the bifurcated waveguide path 25. With this change ofpropagation speed, two lights propagating in respective first and secondbifurcated intermediate paths 24, 25 are modified to be out of phasewith each other. These two lights are merged into one again in theoutlet portion 23. Thus merged light has a modified intensity comparedwith the original one. Thus, the optical modulation can be carried outin the Mach-Zehnder type optical waveguide device.

A monocrystal lithium niobate and a monocrystal lithium tantalaterespectively having a large electrooptical effect are selectively usedin this embodiment as the material constituting the transparentdielectric substrate 22. The monocrystal lithium niobate has an index ofrefraction of 2.29 with respect to an ordinary light. On the other hand,the monocrystal lithium tantalate has an index of refraction of 2.18with respect to the ordinary light.

If there is a certain difference in index of refraction, it is possibleto confine lights or electromagnetic waves in either of two substrateswhich has a greater index of refraction. The transparent dielectricsubstrate 22 used in this embodiment has a greater index of refractioncompared with that of the glass substrate 21 which is generally in arange of 1.4-1.6. Thus, a light entered into the transparent dielectricsubstrate 22 can be confined in this thin layer. The thicknesses of theglass substrate 21 and the transparent dielectric substrate 21 areselected to 1 mm and 7 μm, respectively.

Furthermore, the ridge configuration of this embodiment is effective inthe localized confinement of the propagation of lights orelectromagnetic waves. Because, the portion right beneath the ridge hasa larger effective index of refraction compared with other portion. Inthis embodiment, the specific dimension of the ridge is determined asfollows: A height of the ridge is 3 μm; a width of the waveguide path is10 μm; a length of the bifurcated portion of the waveguide path is 2 cm;and an whole length of the waveguide path is 4 cm.

The ridge configuration is further advantageous in that a center of thepropagation, i.e. a portion at which an intensity of light orelectromagnetic wave is strongest, is positioned almost identically withthe center of the waveguide path formed beneath the ridge. Moreover, theshape of the propagation of lights or electromagnetic waves is similarto a circle. As the inlet and outlet portion 33, 13 of the waveguidepath have the same circular constructions, the coupling efficiency withrespect to an optical fiber, which has generally a circular core(approximately 10 μm in a diameter), is greatly improved.

In fact, a coupling loss with respect to the optical fiber was less than0.3-0.5 dB at one side in the case where an adhesive material whoseindex of refraction is appropriately adjusted is used to connect theoptical fiber and the waveguide device of this embodiment.

Still further, the propagation loss of lights or electromagnetic wavescan be suppressed to the least value because the transparent dielectricsubstrates used in this embodiment are made of pure monocrystal, whichrequires no ion-diffusion treatment. In more detail, a value equal to orless than 0.5 dB/cm was easily attained in the propagation loss.

Furthermore no optical damage was found, although the optical damage wasmeasured by increasing the intensity of incident light from 0 dBm to 20dBm. This good result is considered to be derived from the excellentproperty of the monocrystal substrate having very small electronic trap.

A light used in the measurement was a wavelength of 1.3 μm.

Barium titanate, potassium niobate, and potassium titanophosphate areprospective materials which have similar electrooptical effect and canbe equivalently used as a transparent dielectric substrate of thepresent invention instead of lithium niobate or lithium tantalate. Theindices of refraction of barium titanate, potassium niobate, andpotassium titanophosphate are 2.4, 2.2, and 1.7, respectively.

5.1. EIGHTH METHOD

An example of the method of manufacturing the optical waveguide deviceof the fifth embodiment will be explained below with reference to FIGS.18(a)-18(g).

First of all, the glass substrate 21 and the transparent dielectricsubstrates 22 are prepared. The surfaces of these substrates 21 and 12,which have been ground and finished as mirror surfaces beforehand, aresmoothed and cleaned. In practice, etchant containing hydrofluoric acidis used to clean the surfaces of these two substrates 21 and 22 as shownin FIG. 18(a). Subsequently, the etched surfaces are watered as shown inFIG. 18(b). This watering treatment allows water components such ashydroxyl group, hydrogen, oxygen to adhere on the surfaces of thesubstrates 21, 22. Immediately after the watering treatment, bothsurfaces of the substrates 21, 22 are uniformly connected together asshown in FIG. 18(c) without using any organic adhesive material. Namely,this connection is direct-type connection achieved by the bonding powerof hydroxyl group, hydrogen, oxygen ions adhering on the surface of thesubstrates 21, 22.

In turn, a heat treatment is adopted to the assembly of the substrates21, 22, as shown in FIG. 18(d). The temperature is increased to a rangeof 100°-1100° C. This heat treatment ensures the connecting strength tobe further increased.

Next, in order to form an optical waveguide path, either of thesubstrates, i.e. the transparent dielectric substrate 22 in the drawing,is thinned by the mechanical grind and etching as shown in FIG. 18(e). Athickness of the substrate 22 is reduced to a level of 10 μm. On thesurface of the substrate 22, there is then formed an etching maskcorresponding to the Mach-Zehnder type optical waveguide structure shownin FIG. 14 by using photolithography. Thus, the portion other thanridges 24, 25 of the optical waveguide structure is removed off byetching as shown in FIG. 18(f). The depth formed by this etching is 3μm. Chrome is used as the mask. The etchant used here containshydrofluoric acid. Thereafter, the mask is removed and a pair ofelectrodes 26, 27 is formed by the ordinary photolithography and etchingtechniques as shown in FIG. 18(g). In this manner, according to thiseighth example of the manufacturing method, the optical waveguide deviceof the fifth embodiment can be obtained.

6. SIXTH EMBODIMENT

FIG. 16 shows the sixth embodiment of the present invention, which isapplied to an optical modulator. In FIG. 16, components suffixed by thereference numerals 21-27 are the same as those disclosed in FIG. 14 andtherefore will no more be explained. A reference numeral 29 represents aglass layer having a low melting point, which interposes between theglass substrate 21 and the transparent dielectric substrates 22.

The index of refraction of the glass layer 29 is approximately 1.5. Thethickness of the glass layer 29 is selected to be 100 nm to 1 μm, whichis far thin compared with that of the transparent dielectric substrate22. With this arrangement, it becomes possible to effectively confinelights or electromagnetic waves in the transparent dielectric substrate22 because of the refractive index difference between the first andsecond substrates 21, 22.

Substantially the same function and effect as the fifth embodiment canbe obtained in this sixth embodiment. Especially almost identicalproperties will be obtained, if the thickness of the glass layer 29 isselected to be 0.5 μm.

6.1. NINTH METHOD

An example of the method of manufacturing the optical waveguide deviceof the sixth embodiment will be explained below with reference to FIGS.19(a)-19(g).

First of all, the glass substrate 21 and the transparent dielectricsubstrates 22 are prepared. The surfaces of these substrates 21 and 22,which have been ground and finished as mirror surfaces beforehand, aresmoothed and cleaned, for example, by etching as shown in FIG. 19(a).Subsequently, a glass layer 29 having a low melting point is grown on atleast either of the smoothed and cleaned surfaces of the substrates 21and 22, as shown in FIG. 19(b). A conventional growth method such asspattering is used to form this glass layer 29. The thickness of theglass layer 29 is selected to be 0.3-0.6 μm.

Then, both substrates 21, 22 are uniformly connected together throughthe glass layer 29 as shown in FIG. 19(c) without using any organicadhesive material.

In turn, a heat treatment is adopted to the assembly of the substrates21, 22 and the glass layer 29 interposing therebetween, as shown in FIG.19(d). The temperature is increased to a point nearly equal to themelting point of the glass layer 29. This heat treatment ensures theconnecting strength to be increased. The thickness of the glass layer 29may vary depending on the temperature.

Next, in order to form an optical waveguide path, either of thesubstrates, i.e. the transparent dielectric substrate 22 in the drawing,is thinned by the mechanical grind and etching as shown in FIG. 19(e). Athickness of the substrate 22 is reduced to a level of 10 μm. On thesurface of the substrate 22, there is then formed an etching maskcorresponding to the Mach-Zehnder type optical waveguide structure shownin FIG. 16 by using photolithography. Thus, the portion other thanridges 24, 25 of the optical waveguide structure is removed off byetching as shown in FIG. 19(f). The depth formed by this etching is 3μm. Chrome is used as the mask. The etchant used here containshydrofluoric acid. Thereafter, the mask is removed and a pair ofelectrodes 26, 27 is formed by the ordinary photolithography and etchingtechniques as shown in FIG. 19(g). In this manner, according to thisninth example of the manufacturing method, the optical waveguide deviceof the sixth embodiment can be obtained.

7. SEVENTH EMBODIMENT

FIG. 18 shows the seventh embodiment of the present invention, which isapplied to an optical modulator. In FIG. 18, components suffixed by thereference numeral 21-27 are the same as those disclosed in FIG. 14 andtherefore will no more be explained. A reference numeral 30 representsan intervening layer, which interposes between the glass substrate 21and the transparent dielectric substrates 22.

The intervening layer 30 is constituted by a material selected from agroup consisting of silicon, silicon compound, and metallic oxide suchas polycrystal silicon, amorphous silicon, silicon oxide and siliconnitride. The thickness of the intervening layer 30 is selected to be 100nm to 1 μm, which is far thin compared with that of the transparentdielectric substrate 22. With this arrangement, it becomes possible toeffectively confine lights or electromagnetic waves in the transparentdielectric substrate 22 because of the refractive index differencebetween the substrates 21, 22.

Substantially the same function and effect as the fifth embodiment canbe obtained in this seventh embodiment. Especially almost identicalproperties will be obtained, if the thickness of the intervening layer30 is selected to be 0.5 μm.

7.1. TENTH METHOD

An example of the method of manufacturing the optical waveguide deviceof the seventh embodiment will be explained below with reference toFIGS. 20(a)-20(h).

First of all, the glass substrate 21 and the transparent dielectricsubstrates 22 are prepared. The surfaces of these substrates 21 and 22,which have been ground and finished as mirror surfaces beforehand, aresmoothed and cleaned, for example, by etching. Subsequently, theintervening layer 30 is grown on at least either of the smoothed andcleaned surfaces of the substrates 21 and 22, as shown in FIG. 20(a). Aconventional growth method such as chemical vapor deposition (CVD) orspattering is used to form this intervening layer 30. The thickness ofthe intervening layer 30 is selected to be 0.25-0.5 μm.

Thereafter, both surfaces to be bonded through the intervening layer 30,i.e. the upper surface of the intervening layer 30 and the upper surfaceof the transparent dielectric substrate 21 in the drawing, are adopted ahydrophilic treatment, as shown in FIG. 20(b). Subsequently, thesurfaces to be bonded are watered as shown in FIG. 20(c). Immediatelyafter the watering treatment, both surfaces of the substrate 21 and theintervening layer 30 are uniformly connected together as shown in FIG.20(d) without using any organic adhesive material.

In turn, a heat treatment is adopted to the assembly of the substrates21, 22 and the intervening layer 30 interposing therebetween, as shownin FIG. 20(e). The temperature is increased to a range of 100°-1100° C.This heat treatment ensures the connecting strength to be increased.

Next, in order to form an optical waveguide path, either of thesubstrates, i.e. the second transparent dielectric substrate 22 in thedrawing, is thinned by the mechanical grind and etching as shown in FIG.20(f). A thickness of the substrate 22 is reduced to a level of 10 μm.On the surface of the substrate 22, there is then formed an etching maskcorresponding to the Mach-Zehnder type optical waveguide structure shownin FIG. 17 by using photolithography. Thus, the portion other thanridges 24, 25 of the optical waveguide structure is removed off byetching as shown in FIG. 20(g). The depth formed by this etching is 3μm. Chrome is used as the mask. The etchant used here containshydrofluoric acid. Thereafter, the mask is removed and a pair ofelectrodes 26, 27 is formed by the ordinary photolithography and etchingtechniques as shown in FIG. 20(h). In this manner, according to thistenth example of the manufacturing method, the optical waveguide deviceof the seventh embodiment can be obtained.

As this invention may be embodied in several forms without departingfrom the spirit of essential characteristics thereof, the presentembodiments are therefore illustrative and not restrictive, since thescope of the invention is defined by the appending claims rather than bythe description preceding them, and all changes that fall within meetsand bounds of the claims, or equivalence of such meets and bounds aretherefore intended to embraced by the claims.

What is claimed is:
 1. A method of manufacturing an optical waveguidedevice comprising steps of:smoothing and cleaning surfaces of first andsecond transparent dielectric substrates which are identical with eachother in index of refraction and coefficient of thermal expansion;forming an intervening layer on at least either of said surfaces of thefirst and second transparent dielectric substrates, said interveninglayer having an index of refraction smaller than that of said first andsecond transparent dielectric substrates; adopting a hydrophilictreatment to surfaces to be bonded through said intervening layer;bonding said first and second transparent dielectric substrates throughsaid intervening layer without using organic adhesive material; adoptinga thermal treatment to said first and second transparent dielectricsubstrates having been bonded together through said intervening layer;and forming an optical waveguide path in at least either of said firstand second transparent dielectric substrates.
 2. A method ofmanufacturing an optical waveguide device comprising steps of:smoothingand cleaning surfaces of first and second transparent dielectricsubstrates which are identical with each other in index of refractionand coefficient of thermal expansion; forming an intervening layer on atleast either of said surfaces of the first and second transparentdielectric substrates, said intervening layer having an index ofrefraction smaller than that of said first and second transparentdielectric substrates; bonding said first and second transparentdielectric substrates through said intervening layer without usingorganic adhesive material; applying a voltage to surfaces bonded; andforming an optical waveguide path in at least either of said first andsecond transparent dielectric substrates.
 3. A method of manufacturingan optical waveguide device comprising steps of:smoothing and cleaningsurfaces of first and second transparent dielectric substrates which areidentical with each other in index of refraction and coefficient ofthermal expansion; forming an intervening layer on at least either ofsaid surfaces of the first and second transparent dielectric substrates,said intervening layer having an index of refraction smaller than thatof said first and second transparent dielectric substrates; adopting ahydrophilic treatment to surfaces to be bonded through said interveninglayer; bonding said first and second transparent dielectric substratesthrough said intervening layer without using organic adhesive material;applying a voltage to said intervening layer; and forming an opticalwaveguide path in at least either of said first and second transparentdielectric substrates.
 4. A method of manufacturing an optical waveguidedevice comprising steps of:smoothing and cleaning surfaces of first andsecond transparent dielectric substrates which are identical with eachother in index of refraction and coefficient of thermal expansion;forming a glass layer having a low melting point on at least either ofsaid surfaces of the first and second transparent dielectric substrates,said glass layer having an index of refraction smaller than that of saidfirst and second transparent dielectric substrates; bonding said firstand second transparent dielectric substrates through said glass layerwithout using organic adhesive material; adopting a thermal treatment tosaid first and second transparent dielectric substrates having beenbonded together through said glass layer; and forming an opticalwaveguide path in at least either of said first and second transparentdielectric substrates.
 5. A method of manufacturing an optical waveguidedevice comprising steps of:smoothing and cleaning surfaces of first andsecond transparent dielectric substrates, said second transparentdielectric substrate, made of the same material as said firsttransparent dielectric substrate, having an index of refraction largerthan that of said first transparent dielectric substrate due todifference of contained impurities in said first and second transparentdielectric substrates; adopting a hydrophilic treatment to said surfacesof said first and second transparent dielectric substrates; bonding saidfirst and second transparent dielectric substrates directly withoutusing organic adhesive material; adopting a thermal treatment to saidfirst and second transparent dielectric substrates having been bondedtogether; and forming an optical waveguide path in said secondtransparent dielectric substrate.
 6. A method of manufacturing anoptical waveguide device comprising steps of:smoothing and cleaningsurfaces of first and second transparent dielectric substrates, saidsecond transparent dielectric substrate, made of the same material assaid first transparent dielectric substrate, having an index ofrefraction larger than that of said first transparent dielectricsubstrate; forming a glass layer on at least either of said surfaces ofthe first and second transparent dielectric substrates; adopting ahydrophilic treatment to surfaces to be bonded through said glass layer;bonding said first and second transparent dielectric substrates throughsaid glass layer without using organic adhesive material; adopting athermal treatment to said first and second transparent dielectricsubstrates having been bonded together through said glass layer; andforming an optical waveguide path in said second transparent dielectricsubstrate.
 7. A method of manufacturing an optical waveguide devicecomprising steps of:smoothing and cleaning surfaces of first and secondtransparent dielectric substrates, said second transparent dielectricsubstrate, made of the same material as said first transparentdielectric substrate, having an index of refraction larger than that ofsaid first transparent dielectric substrate due to difference ofcontained impurities in said first and second transparent dielectricsubstrates; forming an intervening layer, including a component selectedfrom a group consisting of silicon, silicon compound, and metallicoxide, on at least either of said surfaces of the first and secondtransparent dielectric substrates; adopting a hydrophilic treatment tosurfaces to be bonded through said intervening layer; bonding said firstand second transparent dielectric substrates through said interveninglayer without using organic adhesive material; adopting a thermaltreatment to said first and second transparent dielectric substrateshaving been bonded together through said intervening layer; and formingan optical waveguide path in said second transparent dielectricsubstrate.
 8. A method of manufacturing an optical waveguide devicecomprising steps of:smoothing and cleaning surfaces of a glass substrateand a transparent dielectric substrate, said transparent dielectricsubstrate having an index of refraction larger than that of said glasssubstrate; adopting a hydrophilic treatment to said surfaces of saidglass substrate and transparent dielectric substrate; bonding said glasssubstrate and transparent dielectric substrate directly without usingadhesive material; adopting a thermal treatment to said glass substrateand said transparent dielectric substrate having been bonded together;and forming an optical waveguide path in said transparent dielectricsubstrate.
 9. A method of manufacturing an optical waveguide devicecomprising steps of:smoothing and cleaning surfaces of a glass substrateand a transparent dielectric substrate, said transparent dielectricsubstrate having an index of refraction larger than that of said glasssubstrate; forming a glass layer, having a melting point lower than thatof said glass substrate, on at least either of said surfaces of theglass substrate and transparent dielectric substrate; bonding said glasssubstrate and transparent dielectric substrate through said glass layerwithout using organic adhesive material; adopting a thermal treatment tosaid glass substrate and said transparent dielectric substrate havingbeen bonded together through said glass layer; and forming an opticalwaveguide path in said transparent dielectric substrate.
 10. A method ofmanufacturing an optical waveguide device comprising steps of:smoothingand cleaning surfaces of a glass substrate and a transparent dielectricsubstrate, said transparent dielectric substrate having an index ofrefraction larger than that of said glass substrate; forming anintervening layer, including a component selected from a groupconsisting of silicon, silicon compound, and metallic oxide, on at leasteither of said surfaces of the glass substrate and transparentdielectric substrate; adopting a hydrophilic treatment to surfaces to bebonded through said intervening layer; bonding said glass substrate andtransparent dielectric substrate through said intervening layer withoutusing organic adhesive material; adopting a thermal treatment to saidglass substrate and said transparent dielectric substrate having beenbonded together through said intervening layer; and forming an opticalwaveguide path in said transparent dielectric substrate.